This invention relates to impregnated cathodes intended for thermionic emission, and is more particularly directed to a cathode in which a body or substrate of highly porous metal (typically tungsten) is treated with a barium-containing reagent, so that barium at the surface of the metal substrate reduces the cathode work function.
Present day thermionic emitter cathodes employ a work-function-lowering mechanism to achieve high current densities at moderate operating temperatures. One such mechanism involves an electropositive monolayer absorbed onto the surface of a metallic conductor. This permits relatively high current densities, on the order of 2 amperes per square centimeter, at a moderate operating temperature on the order of 1300 K. (i.e., 1000.degree. C.). Unfortunately, even at these operating temperatures, there is significant evaporation of the surface monolayer. To keep this evaporation from resulting in early degradation of the cathode, replenishment of the monolayer is necessary; this is typically accomplished by dispersing alkaline earth oxides within a porous metallic matrix of the cathode structure, and at operating temperatures, the impregnant "dispenses" its active element to replenish the monolayer.
Impregnated cathodes, as typified by so-called "dispenser" cathodes, comprise a tungsten matrix in which barium adheres to the surface tungsten layer. Ideally, the barium becomes vertically oriented over a surface oxygen atom on the tungsten. This forms a structure which resembles BaO. In operation, the elevated temperature of the cathode causes the barium layer to boil off. As the barium evanesces, other barium is liberated from within the matrix and rises to the surface to replace the boiled-off barium. However, to form this typical dispenser cathode, barium oxide cannot simply be mixed into a porous metal matrix. It has been found that molten barium oxide, if impregnated directly onto the tungsten, tends to poison the cathode material. Consequently, a typical dispenser cathode is formed by impressing a nickel powder and an alkaline earth carbonate, such as CaCO.sub.3, together with barium carbonate into the pores of the metal matrix. The material is then heated so as to break the carbonates into oxides and evolve CO.sub.2, leaving barium compounds buried within the matrix, and having a layer of BaO on the surface. Unfortunately, the continuous evolution of CO.sub.2 gas in this type of dispenser cathode presents some difficulties in maintaining high vacuums within the electron tubes that employ this type of cathode. Furthermore, the dispenser cathode requires an extremely high operating temperature, i.e., 1100 degrees C. or higher, to effect the dispensing of the buried barium to the surface.
While the chemistry of a typical dispenser cathode has not been completely understood, it is thought that the operating heat creates hypothetical free barium which rises to the surface. A series of complex chemical reactions, which involve both the dispersed supporting alkaline earth oxides and the metal of the tungsten matrix, creates free barium (hypothetically) as the surface barium layer evanesces. These reactions are typically initiated at a cathode activation temperature of 1250.degree. C. (1523 K.) and are sustained at an operating temperature of 1000.degree. C. (1273 K.). Ideally, the captive barium would be liberated and diffuse to the surface from pores in the matrix until all the barium was depleted. However, this does not occur in real-world dispenser cathodes.
One realization that was never made previously concerning barium-tungsten dispenser cathodes was that the Ba-O-W ideal vertical structure would be a two-dimensional site group, rather than a realistic three-dimensional crystallographic point group. Consequently, the prior art could not explain failures and shortcomings of dispenser cathodes.
A number of cathodes of this type have been previous proposed. Levi et al. U.S. Pat. No. 2,700,000 relates to a barium-aluminate-treated cathode, and Levi U.S. Pat. No. 3,201,639 relates to a barium and alkaline earth oxide impregnated cathode. Falce U.S. Pat. No. 4,165,473 describes a dispenser cathode having a tungsten and iridium matrix that is impregnated with BaAl.sub.2 O.sub.4. Henderson et al. U.S. Pat. No. 3,134,924 describes an oxide cathode, in which nickel and thorium oxide are dispersed within a matrix that is deposited on a metal substrate. In Toguchi et al. U.S. Pat. No. 4,518,890, an impregnated cathode is formed of scandium an aluminum oxide in a porous tungsten body impregnated with barium oxide.
All of the above-mentioned cathodes have drawbacks. All of these cathodes require barium oxide deposited on the surface, and barium oxide is volatile at typical cathode operating temperatures of 1000 degrees C. or higher. Even though replenished from within, the available barium metal is all eventually consumed. Further, far less than the optimum surface area of the cathode can be used, which leads to rather low efficiency. In the production of the dispenser cathodes, the metal matrix pores are plugged up with the barium and alkaline earth carbonate material. Filling the pores limits the surface area, and thereby limits the cathode current available for the cathode of a given diameter, as compared with the relatively larger amount of surface area that would be available if the pores were left open, by a factor of about 10.sup.3.
In addition, the work function for a Ba-O-W cathode surface is still somewhat high, requiring cathode operating temperatures in the range of 1000 degrees C.-1300 degrees C. These higher temperatures are also required to obtain the dispensing effect for replenishing the barium.
Previous dispenser and other impregnated cathodes are typified by patchiness of emission. That is, much of the impregnant material on the surface of the matrix occurs in massive deposits several microns thick and many microns across; these deposits tend to remain after cathode activation and block much of the surface area of the matrix. Moreover, as the cathode ages and the barium evanesces, the barium is not replaced evenly over the surface of the cathode, but tends to cluster near dispensing sites.
An ideal impregnated thermionic cathode would have the characteristics of copious electron emission, supplying an unlimited current density which, in theory, would never reach saturation, would have a low activation temperature, a low operating temperature thereby prolonging its operating life indefinitely, no loss of its key constituents during its lifetime, and be reproducible under controlled conditions so that the cathode performance is consistent and predictable. However, in state-of-the-art cathodes, current density is rather limited, activation of the cathode requires temperatures typically in excess of 1200 degrees C., operating temperatures is typically above 800 degrees C., usually much higher. The barium or barium oxide evaporation rates tend to be significant, and process conditions have been difficult to control, leading to poor reproducibility and inconsistent results from one cathode to the next.